A mathematical model was used to derive operational equations for examining incorporation and turnover of fatty acids within individual brain phospholipids, under in vivo conditions in animals and humans. In rats, the model was combined with quantitative autoradiography and biochemical analysis to examine these parameters with the saturated (9,10--3H] palmitic acid (3H-PAM) and the unsaturated [1-14C]arachidonic (14C-AA) and (1- 14C]docosahexaenoic acids (14C-DHA). We have refined the model by thorough analysis of phospholipid metabolic precursor pools and defined their rate of turnover and the turnover of fatty acids in membrane phospholipids. We have applied the model to a number of systems in which membrane homeostasis can be either acutely or chronically perturbed. Administration of serotonergic agonists indicated involvement of [14C]AA but no [3H]PAM in second messenger systems. Incorporation of the three tracers was increased threefold to fivefold compared to normal brain by cerebrally implanted Walker 256 carcinomasarcoma cells in rats. Fractionation of brain membranes after arecoline stimulation indicated rapid turnover of arachidonate in synaptosomes. Chronic treatment of rats with lithium chloride was shown to alter the metabolism of the three tracers in brain phospholipid. Seizures induced in a kindled rat model showed increased incorporation of [3C]PAM in specific brain areas. Rates of fatty acid incorporation were shown by theory and experiment to be independent of cerebral blood flow. An inhibitor of fatty acid oxidation increased the fraction of labeled palmitate that entered brain lipids. We have used the model to define radiolabeled fatty acid imaging of the brain by positron emission tomography (PET). [1-11C] fatty acids were synthesized and gave incorporation coefficients in monkeys comparable to those in rats. PET can be use to image normal and pathological brain in vivo.